In a communication system mounted in a vehicle, a communication system that uses a PWM code as a transmission code has been known (see, for example, SAE International J1850). In a case where a high level of a signal level on a transmission line is set as recessive and a low level of the signal level on the transmission line is set as dominant, the transmission line is configured so that the signal level on the transmission line becomes dominant when any one of nodes outputs a signal of dominant. In this case, it can be considered that the communication system is configured so that a waveform of the transmission code is determined by superimposing the signals output from the respective nodes.
In other words, a PWM code of a small ratio of the low level is associated with logic 1 and a PWM code of a large ratio of the low level is associated with logic 0, and one of nodes (master node) outputs the PWM code of logic 1 in a bus idle state in which any node does not communicate. A node (slave node) other than the master node outputs such a signal that the transmission code on the transmission line becomes a desired PWM code when superimposed on the PWM code of logic 1 output from the master node.
Specifically, for example, when the slave node outputs the PWM code of logic 1, the PWM code of logic 1 can be actualized by outputting a signal at the high level over the whole period of the code. On the other hand, when the slave node outputs the PWM code of logic 0, the PWM code of logic 0 can be actualized by outputting a signal that rewrites a part of the PWM code of logic 1 output from the master node from the high level to the low level.
In general, a driver circuit of a node is formed using a transistor that connects and disconnects the transmission line and the ground line. In other words, the transistor is turned off when an output signal of the node is set to be recessive, and the transistor is turned on when an output signal of the node is set to be dominant.
Thus, in a case where the PWM code is rewritten to logic 0 by the slave node as described above, an output signal of an driver circuit of the master node is changed from the low level to the high level when an output signal of a driver circuit of the slave node is maintained at the low level. Then, at the moment, electric current flows from the master node to the driver circuit of the slave node that rewrites the code (i.e., the slave node that outputs the signal at the low level), and noise may be generated due to a rapid current change.